Arianne Cohen and her husband have a happy—and open—marriage. Why they decided to try the brave new world of polyamory—and how they make it work.

I'M UNDER THE COVERS WITH TIM, a burly video artist, ostensibly watching Mad Men on Netflix but actually making out. This happens most Tuesdays. My phone buzzes with a text from my husband, Thomas: "Have a good night, babe. Say hi to Tim."

Thomas knows what's going on. We've been married for a year and in an open relationship for five, meaning that we're free to see other people. He likes not feeling shackled—before me he was in a 12-year monogamous marriage, and now he prefers freedom—and I enjoy the occasional variety, while improving my relationship skills tenfold. (When multiple men complain that you're not communicating well, you know it's time to change. It's like relationship-feedback boot camp.)

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It wasn't a smooth road to our current sexy landing. For starters, staying open was Thomas' idea. We met at a party five years ago near his Brooklyn apartment when I was visiting from Portland, Oregon; during our third long-distance rendezvous, he announced, "I think we should keep it open for now." Open is not the word a girl wants to hear from the most promising partner she's met after five years of dating duds. My first thought was an image of him in flagrante with another woman, which led to my second thought: No way. Wasn't I enough?

But I understood Thomas' request. Though it stung—like every woman, I like to hear that I'm the best, prettiest, and only game in town—there was something appealingly honest in his request. He didn't want to miss chance encounters and playful fun while I was across the country, and frankly, neither did I. We were crazy about each other but also 3,000 miles apart, and intellectually, I knew that enforcing limitations on guys—or anyone, really—doesn't work. I'd never been particularly sexually adventurous, but something about his truthfulness felt right. "OK," I reluctantly agreed. "I'll try it."

So I was rather stunned when six months later I found myself in the most sexually and emotionally fulfilling period of my life. Just being with other men, without expectations for a "future," worked. I saw Thomas every few weeks and dated lazily. It was surprisingly easy. I met two flings at parties and another on a writing assignment, and saw them whenever it made sense—sometimes a last-minute tryst, sometimes not for months. All three were outside my usual stable-life-partner type: One was an enchanting artist who was foreclosing on his house; another, a selfish guy who was a dependably great time; the third, an older kinky man who deliciously tied me up. Each dalliance had all the heat of a taboo affair, minus the burn of actual cheating. None would have happened had Thomas not been in the picture; it was the first time I'd stopped fixating on finding my one-and-only perfect soul mate.

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DEALING WITH THOMAS' flings was more painful. When I was alone many nights in Portland, my mind self-immolated while imagining his bedroom activities. I thought my lovers were delightful but couldn't cope with his. Our agreement was to tell the truth when asked, but asking made me nauseous. Instead, I imagined, assumed, and felt sick. On one trip, I scanned his cell phone while he slept and found a "Thank you for last night" text from 48 hours earlier.

That "thank you" propelled two days of bathroom crying and obsessing over what all the "thanking" was for. By the time Thomas said, "Hey, so let's talk about who we've been seeing," I felt like an emotional trauma victim. He was honest: "She's a friend. We had sex. It was OK." I thought, Oh. Well, that's not so bad. His answers squared with my snooping and were mundane compared with my mind's spirals. I didn't know it then, but "open relationship" really means "honest, direct communication." It's less about sex and more about being emotionally bare, which is the only way through it.

Three weeks after the talk, while Thomas snored next to me, I couldn't sleep. I poked him. "This is when I usually go through your phone," I blurted out. I poured out my thoughts: that I loved sleeping around but was a jealous mess; I was a big snoop; I worried that he'd fall in love with someone else and I'd never recover. His insecurities—mainly a nagging worry that I'd see men and not tell him—were easier to ameliorate. I just always told the truth.

"Aww, babe," Thomas said, pulling me in. He listed all the reasons he loved me and assured me I had nothing to worry about. "The other girls just make me miss you," he said. It wasn't a line; I knew what he meant. Other lovers fill your life with cuddles and laughs, but they are not soul mates.

He moved in with me in Portland after a year, and neither of us wanted to give up our extracurriculars, so we decided that Tuesday would be our date night with outside partners. The idea was to give each other time to plan ahead. We initially had many rules, all of which boiled down to: Communicate before you do anything. A blindsided partner is a sad partner.

During weeks when only one of us had a date, our plan didn't always work. After a miserable Tuesday night alone spent eating an entire box of cereal, I tried new strategies for my vacant nights—dinner with friends, a work project, a good book. I found that when I genuinely enjoyed my Tuesdays, I didn't even think about Thomas' whereabouts or worry about waiting up for him. And when I woke up one Wednesday morning to my own "Thank you for last night," I sighed happily. Deep down, underneath all my neuroses, I wanted Thomas to enjoy the same dalliances.

We both learned basic non-monogamy manners through trial and error. Springing an unannounced date on a busy, stressed-out partner is a bad idea, as we learned when Thomas wandered home at 1 a.m. with a woman in tow in the midst of my work all-nighter. And if one partner feels like he or she is not getting attention at home, sleeping with someone else is going to backfire—badly. (We discovered this after I didn't see Thomas for a week, then went on a date; a huge fight ensued.) We decided that we wanted full veto power over each other's partners, instituting a "no fly" list after I started seeing a guy whom Thomas described as "nothing but trouble." Low-drama partners are the key to polyamory. One night four years in, I realized that I couldn't think of a single secret I had kept from him.

Our monogamous friends and family ask the usual questions, like how, why, and "Don't you feel hurt?" (No. Do you?) They eventually lose interest; the few who are judgmental let their feelings be known. My fears about Thomas' lovers fell away because he dates women who are my opposite. He already has one type A writer in his life, so he comes home with hippie chicks with unusual dietary needs and loosely defined "jobs." I meet them for coffee—or, well, herbal tea—because putting a face on the beast diffuses the angst. I'm not worried. His dates provide sexual and spiritual and intellectual experiences that I can't, and they're useful. I say things like, "You know, Jenny actually cares about this topic."

Since we're committed life partners, last year we tied the knot. Now our rules are looser; we just know how to not hurt each other, and we're better at damage control. A therapist recently told me that open relationships only work if the partners are doing it because they care about their partner, not because they want to sleep around. It's true. My husband is a better human being on a long leash, and so am I.

I still search his phone. I just like to. Only now I say, "Hey, sexy, I'm looking through your phone." He doesn't blink. If a message gives me a pang, we talk about it. Openly.